Thanks. That helped. I did have to use two separate usb-sticks. I obviously couldn't load the *.iso onto another usb-stick and then upgrade my initial one, since that one was too small. So I saved the db file on another pc in the network through the WebGUI, flashed the 9.3 beta *.iso onto one new usb-stick, then installed FreeNAS onto a second (16gb) one. That took quite a while - for those trying this, don't get worried if the console doesn't display anything for 2-3 minutes - but it did work perfectly. I then reimported the db configuration through the webgui (I had been allocated a new ip-address) and rebooted - and to my great relief everything worked just fine. It also went back to the original ip-address and login credentials, so that all the bookmarks and settings on my networked devices are still valid. All jails (excepts for minidnla for some reason) work perfectly, nothing got lost. I then also updated my volume to the new zfs - as I was asked to do - and so I now have a wonderfully green 'Alert' button and a fully functioning FreeNAS 9.3, which hopefully, from now on, will also always be automatically upgradable...
A great thanks to all developers!